In a long-haul optical fiber communication system for transmitting signal light of C band or L band, an optical fiber amplifier to which a rare earth-element such as erbium (Er) is doped is used as an optical amplifier that amplifies the signal light. A gain spectrum of an amplifier (Erbium-doped fiber amplifier, hereinafter referred to as “EDFA”) containing an optical fiber doped with erbium (Erbium-doped fiber, hereinafter referred to as “EDF”) as an optical amplifying medium has a peak in the wavelength of 1.53 μm band. The bit error rate increases and the performance of the transmission system deteriorates due to non-flatness of this gain spectrum. As a component for avoiding such performance deterioration, a fiber Bragg grating (hereinafter referred to as “FBG”) that is a gain flattening device, in particular, a slanted fiber grating (hereinafter referred to as “SFC”) have been developed.
Meanwhile, in recent years, as a technique of greatly increasing the transmission capacity per optical fiber, a long-haul optical fiber communication system has been proposed. The long-haul optical fiber communication system performs spatial multiplexing signal light transmission, using a multicore optical fiber (hereinafter referred to as “MCF”) constituted by a plurality of light guiding structures each including a core and a single common cladding surrounding the plurality of light guiding structures, as an optical transmission line. From this, the importance of multicore EDF (MC-EDF) and multicore SFG (MC-SFG) is increasing.
Technologies of manufacturing a gain flattening device and the like using a single-core optical fiber are described in Patent Documents 1 and 2. An optical fiber including a core or a cladding comprised of silica glass in which a photosensitive material (for example, GeO2 or B2O3) is doped is irradiated with ultraviolet light with spatially modulated intensity in an axial direction of the core, whereby a grating having refractive index distribution according to intensity distribution of the ultraviolet light in the axial direction of the core can be written. As the ultraviolet light, a second harmonic wave of argon ion laser light (244 nm), KrF excimer laser light (248 nm), a fourth harmonic wave of YAG laser light (265 nm), a second harmonic wave of copper vapor laser light (255 nm), or the like is applicable.
As a method for irradiating an optical fiber with ultraviolet light with spatially modulated intensity along the axial direction of the core, there are a phase mask method for causing ±1 order diffracted light generated using a chirped grating phase mask to interfere with each other, a method for directly exposing the optical fiber with the laser light, and a two-light flux interference exposure method for causing two of causing two branched light beams to interfere with each other after branching laser light into two. Among the aforementioned methods, the phase mask method can more easily manufacture the grating with more favorable reproducibility than other methods.
A technology for manufacturing an MC-SFG is described in Patent Document 3. In the manufacturing technique described in Patent Document 3, a plurality of light guiding structures of an MCF is simultaneously irradiated with ultraviolet light with spatially modulated intensity after the surroundings of the MCF are filled with matching oil, whereby gratings are simultaneously formed in the plurality of light guiding structures. The reason why the surroundings of the MCF are filled with the matching oil is to compensate for the condensing effect due to the fact that the MCF has a cylindrical shape. In this manufacturing technology, since the gratings are simultaneously formed in the plurality of light guiding structures of an MCF, the manufacturing time can be reduced. In addition, uniformity of characteristics of the gratings respectively formed in the plurality of light guiding structures is expected.